Wireless network (mobile communication network) systems are typified by a network system using a wireless LAN (IEEE 802.11) as illustrated in FIG. 1 (wireless LAN system), and a cellular network system as illustrated in FIG. 2 (3GPP).
As illustrated in FIG. 1, for example, the wireless LAN system includes a media server, a radio network gateway connected to the media server via a link (network), and an access point (AP) accommodated in the radio network gateway. A terminal (such as personal computer (PC) or personal digital assistant (PDA)) is connected to the AP via a wireless link (wireless transmission path). The section between the AP and the terminal constitutes a wireless section of a data transmission path.
As illustrated in FIG. 2, for example, the cellular network system (cellular phone network system) includes a server, a gateway (GW: router) connected to the server via a link, a switching device (xGSN) connected to the GW via an IP network such as the Internet, a radio network controller (RNC) connected to the switching device via a link, and a base station device (Node B or BS) connected to the radio network controller via a link. A mobile terminal (user equipment or mobile node) is connected to the base station device via a wireless link (wireless transmission path). The section between the mobile terminal and the base station device constitutes a wireless section of a data transmission path.
With regard to communication (wireless communication) in the wireless section, as effective techniques for reducing transmission errors of data to be received, there are provided interleaving for data to be transmitted, and error correction using forward error correction (FEC). Those are employed in, for example, the cellular network system.
FIG. 3 is a diagram illustrating a general protocol stack in a case where the IEEE 802.11 is applied to a wireless network. There is provided H.264/AVC as one of video compression standards to be applied to real-time streaming services applicable to the wireless network system. In the H.264/AVC, in order to provide streaming services, a real-time transfer protocol/RTP control protocol (RTP/RTCP) is used as a lower-layer protocol thereof. The interleaving may be performed on real-time data to be transferred in conformity to the RTP/RTCP (RTP packet) in consideration of the wireless environment (see Non-patent Document 1, in particular, section 12.6 and section 13).
Accordingly, the upper-level transmission protocol of the RTP/RTCP may provide robust data transmission even when the lower-layer link is a wireless link.
Streaming data is used for services that require instantaneous reproduction, such as live programs, live broadcast, and video teleconferences. In other services such as on-demand video program distribution, streaming data is allowed to be buffered temporarily. For example, with regard to a reproduction timing on a timeline (time base) at the time of data reproduction, reproduction is started after a certain amount of data is buffered. Further, services such as TV program distribution do not require all pieces of data transmitted from the media server to reach the receiver side at the time of data reproduction.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-536097
Patent Document 2: Japanese Laid-open Patent Publication No. 2002-198946
Patent Document 3: Japanese Laid-open Patent Publication No. 06-181567
Patent Document 4: Japanese Laid-open Patent Publication No. 2000-156646
Non-patent Document 1: RFC 3984 RTP Payload Format for H.264 VIDEO
However, the wireless environment largely changes as compared with a fixed line. Therefore, the cycle in which interleaving is performed (interleaving cycle) does not always suit to the wireless environment, which may cause a case where a transmission error reduction effect obtained through the interleaving is not exerted sufficiently.
Streaming data (real-time data) to be transmitted in a real-time service or other such data has much less opportunities for recovery thereof through retransmission performed in a case where a transmission error has occurred, as compared with a non-real-time service. Therefore, a concealment technology (error correction technology) to be used in the real-time service at the time when a transmission error has occurred is more important than in the case of the non-real-time service.
In wireless transmission, in a case where a situation has changed from an error free environment to an error prone environment, for example, in order to maintain constant quality of the transmission path, that is, achieve quality of the wireless transmission path in which error correction is possible, redundant codes constituting error correction codes are increased and/or a modulation scheme is changed (for example, the modulation scheme is changed from QPSK to BPSK) as well as transmission power control, to thereby reduce a transmission bandwidth for an upper-level application.
Such a situation that the transmission bandwidth is reduced is inconvenient for real-time data. Specifically, the reduction in transmission bandwidth due to deterioration in transmission quality has a fear that a time of arrival of data to be transmitted within a certain time period at its destination is extremely delayed. In addition, there is a fear that data is continuously discarded at the destination due to the deteriorated quality of the transmission path, before the bandwidth is adjusted in the wireless section.
The situation described above, in which the arrival time is delayed or data is continuously discarded, also results from hard handover. FIG. 4 is an explanatory diagram illustrating occurrence of discard and dwell of data at the time of hard handover.
Specifically, in a case where a bandwidth usable at a handover destination is narrower than a bandwidth used at a handover source, temporary data missing occurs at the time of handover. In other words, when a path of data for a certain area is switched to a path for another area, data for which the switching is in progress may be lost. When such data loss occurs, reproduction is not performed on the destination side until the lost data is compensated through retransmission, with the result that data dwell (long-time buffering) may occur. In other words, there is a fear that the data loss significantly influences reproduction of real-time data in a specific time period.
As described above, in the mobile communication network in which the communication environment changes variously, the interleaving cycle for performed interleaving, which is uniquely determined in conformity to, for example, the RFC 3984, may not always be an appropriate cycle.